1. Field of the Invention
The present invention relates to a flux used for brazing aluminum and an aluminum alloy (both referred to as an aluminum material hereinafter). More particularly, the present invention relates to a flux used for brazing an aluminum material with a high magnesium content, said flux being a complex compound containing cesium (Cs) as a complex salt and crystalline aluminum hydroxide and/or aluminum oxide.
2. Description of the Related Art
The brazing of an aluminum material is conventionally accomplished with an aluminum-silicon eutectic alloy as a brazing filler metal, which has a slightly lower melting point than has the aluminum material. To achieve the good joining of the parts of an aluminum material, the brazing filler metal is applied to the joint together with a flux which cleans the aluminum material of surface oxide films and other contaminants. One of the new fluxes is a noncorrosive flux which is a KF-AlF.sub.3 complex (potassium fluoroaluminate). A disadvantage of this flux is that it melts at a eutectic point of KF-AlF.sub.3 which is 560.degree. C., and hence it should be used with a brazing filler metal having a melting point higher than 560.degree. C. by tens of degrees. The brazing filler metal and flux having such a high melting point necessitate the brazing temperature to be high and present difficulties in controlling the brazing temperature, particularly in the case where heating is by manual brazing torches (acetylene flame). In other words, brazing with the KF-AlF.sub.3 flux needs a great deal of skill. Another disadvantage of this flux is that it is not very effective in the brazing of a magnesium-containing aluminum material. It is said that this flux cannot be used for the brazing of an aluminum material containing more than 0.4 wt % of magnesium.
To eliminate the disadvantages involved in the related art technology, there has been proposed a brazing flux which is composed of 100 wt % of potassium fluoroaluminate (or potassium fluoroaluminate and aluminum fluoride) and 5-15 wt % of aluminum ammonium fluoride, said potassium fluoroaluminate containing 60-50 wt % of aluminum fluoride and 40-50 wt % of potassium fluoride. It is said that this brazing flux can be applied to an aluminum material containing magnesium up to about 2 wt %. (See Japanese Patent Laid-open No. 184490/1985.) This brazing flux still suffers from a disadvantage or having a melting point as high as 569.degree.-580.degree. C. and giving off a large amount of harmful fumes of ammonium fluoride (NH.sub.4 F) during brazing. The harmful fumes present a serious problem associated with safety and pollution.
To eliminate the disadvantages involved in the related art technology, the present inventors proposed a brazing flux which is cesium fluoroaluminate containing aluminum fluoride and cesium fluoride in a molar ratio of from 67:33 to 26:74, or is a mixture composed of said cesium fluoroaluminate and aluminum fluoride. (See U.S. Pat. No. 4,689,092.) This brazing flux is of practical use because it melts at a lower temperature (440.degree.-460.degree. C.) than KF-ALF.sub.3 flux by about 120.degree. C. In addition, it can be applied to a magnesium-containing aluminum material. However, although it is superior to the KF-ALF.sub.3 flux when applied to an aluminum material with a high magnesium content, it does not fully exhibit its performance if the magnesium content exceeds 1 wt %.
The above-mentioned fluxes easily oxidize and deteriorate unless they are used under strictly controlled conditions; in other words, they are not well suited for torch brazing. (Note that the one disclosed in Japanese Patent Laid-open No. 184490/1985 has a melting point in a narrow range of 569.degree.-580.degree.C. and the one disclosed in U.S. Pat. No. 4,689,092 has a melting point in a narrow range of 440.degree.-460.degree. C.)
The problems in the related art technology have also been approached with another brazing flux for an aluminum material and a process for its production. (See U.S. Pat. No. 4,923,530). This flux is produced with an amorphous aluminum hydroxide and has a composition of M.sub.x AlF.sub.y O.sub.z (where x =0.5-2.0, y=1.5-4.8, z=0.1-1.0, and M denotes Li, Na, K, Rb, or Cs). This flux has good stability in suspension in addition to a low melting point like the flux disclosed in U.S. Pat. No. 4,689,092. Further, this flux is easy to produce and economically advantageous. However, with M being cesium, this flux has a melting point in a narrow range of 410.degree.-440.degree.C. Therefore, it easily oxidizes and deteriorates unless it is used under strictly controlled conditions; in other words, it is not well suited for torch brazing.
The present inventors have proposed a non-corrosive brazing flux of CsF-AlF.sub.3 -KF, which has a controlled melting point within a broad range of 440.degree.-580.degree. C. and can be effectively applied to a magnesium-containing aluminum material. (See U.S. Pat. No. 4,670,067.) A disadvantage of this flux is that it contains K and active F which form high-melting fluorides such as KMgF.sub.3 and MgF.sub.2 to reduce the flowability of the brazing filler metal. Therefore, it is not well suited for the brazing of an aluminum material containing more than 1 wt % of magnesium. This is true particularly in the case where it is applied to the atmospheric torch brazing of a magnesium-rich aluminum material. In this case it reduces the flowability of the brazing filler metal and prevents it from completely running between the objects to be joined.
In order to solve the above-mentioned problems involved in the related art technology, the present inventors carried out a series of researches, which led to the present invention.